1. Field of the Invention
This invention is concerned with a process for catalytically dewaxing a hydrocarbon oil. In particular, it is concerned with dewaxing a petroleum oil such as a whole crude, a reduced crude, or a distillate fraction thereof by contact with a zeolite catalyst exemplified by ZSM-5 associated with a nickel-tungsten hydrogenation component. It is further concerned with a process for manufacturing low pour point distillate fuels. It is still further concerned with a catalytic dewaxing process for manufacturing a high V.I. distillate lubricating oil stock of low pour point and good stability. Products produced by the method of this invention need less severe or no hydrofinishing, or less amounts of stabilizing additives, since they are inherently of improved stability.
2. Prior Art
Catalytic dewaxing of hydrocarbon oils to reduce the temperature at which separation of waxy hydrocarbons occurs is a known process. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113, which described dewaxing followed by hydrofinishing.
Reissue Pat. No. 28,398 to Chen et al, reissued Apr. 23, 1975, describes a process for catalytic dewaxing with a catalyst comprising a zeolite of the ZSM-5 type. A hydrogenation/dehydrogenation component may be present.
A process for dewaxing a gas oil is described in U.S. Pat. No. 3,956,102 issued May 11, 1976.
A mordenite catalyst containing a Group VI or a Group VIII metal is used to dewax a low V.I. distillate from a waxy crude, as described in U.S. Pat. No. 4,110,056 issued July 11, 1978.
U.S. Pat. No. 3,755,138 to Chen et al describes a process for mild solvent dewaxing to remove high quality wax from a lube stock, which is then catalytically dewaxed to specification pour point.
The patents and publications cited above are illustrative of the dewaxing art as applied to various hydrocarbon oils, including crude peteroleum. A number of other patents and publications are known, but applicant is not aware that any of these is particularly pertinent to his invention.
Petroleum products generally are required to have some specified stability properties consistent with intended use. This requirement is in addition to the requirement that the pour point or freeze point for certain oils be low enough to cause no flow problem. Thus, hydrocarbon oil products intended for use as jet fuel, lubricating oils, or as fuel oil, are sometimes treated with additives to improve oxidation stability, reduce deposit formation, or both. In other cases these oils are "hydrofinished" for the same reason. In still other cases both "hydrofinishing" and additives may be required to achieve the required stability.
Catalytic dewaxing followed by catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938 issued July 15, 1975.
Each of the patents and publications cited above is incorporated herein by reference.
As is evident from the foregoing references, problems associated with waxy constituents in a hydrocarbon oil may occur with crude oil, jet fuel, home heating oil, and the distillate or residual fractions used to prepare lubricants. The resistance to low temperatures required for each of these products depends on the type of exposure encountered. For example, the permissible pour point of a crude to be transported by pipeline in Saudi Arabia may be considerably higher than that of a crude to be transported in Alaska. Furthermore, a jet fuel is required not to separate waxy material at a temperature above about -40.degree. F., for example, while a pour point for the common home heating oil of about +20.degree. F. would be satisfactory in a temperate climate. As a result of this diversity of requirements, a number of specifications have been developed which apply to different products. Some of these specifications, and the method by which they are determined, include: Pour Point, determined by ASTM Standard D97; Cloud Point, ASTM D-2500; Freeze Point, ASTM D-2836; and Cold Filter Plugging Point (CFPP), DIN 51428, SIS- 155122, and AFNOR 549 Standards, Througout this specification, it will be understood that when the term Pour Point is used, the comparable Cloud Point, Freeze Point, or CFPP value may be substituted when appropriate.
Lubricating oils for use in automotive and aircraft engines must not vary too much in viscosity as temperature changes. This requirement is in addition to the requirements for satisfactory pour point and stability. Since the present invention is useful in the preparation of high quality lubricating oils, a brief summary of this highly developed and complex art is now given.
Although the broad principles involved in refining of lubricating oils are qualitatively understood, the art is encumbered by quantitative undertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 450.degree. F., the molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structures and structure types. This complexity and its consequences are referred to in "Petroleum Refinery Engineering", by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition), relevant portions of this text being incorporated herein by reference for background.
In general, the basic notion in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as, for example, appropriate viscosity, oxidation stability, and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining, and dewaxing, which basically are physical separation processes in the sense that if all the separated fractions were recombined one would reconstitute the crude oil.
A refined lubricant stock may be used as such as a lubricant, or it may be blended with another refined lubricant stock having somewhat different properties. Or, the refined lubricant stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives, and V.I. improvers. As used herein, the term "stock", regardless whether or not the term is further qualified, will refer only to a hydrocarbon oil without additives. The term "raw stock" will be used herein to refer to a viscous distillate fraction of crude petroleum oil isolated by vacuum distillation of a reduced crude from atmospheric distillation, and before further processing, or its equivalent. The term "solvent-refined stock" will refer to an oil that has been solvent refined, for example with furfural. The term "dewaxed stock" will refer to an oil which has been treated by any method to remove or otherwise convert the wax contained therein and thereby reduce its pour point. The term "waxy", as used herein will refer to an oil of sufficient wax content to result in a pour point greater than +25.degree. F. The term "stock", when unqualified, will be used herein generically to refer to the viscous fraction in any stage of refining, but in all cases free of additives.
Briefly, for the preparation of a high grade distillate lubricating oil stock, the current practice is to vacuum distil an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450.degree. to 1050.degree. F. After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g. furfural, phenol, or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example by admixing with a solvent such as a blend of methyl ethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes which are then separated from the dissolved oil, and the dewaxed raffinate is then recovered by removal of the solvent.
Other processes such as hydrofinishing or clay percolation may be used if needed to reduce the nitrogen and sulfur content or improve the color and stability of the lubricating oil stock. Bright stocks are often deasphalted, e.g. by treatment with propane.
Viscosity Index (V.I.) is a quality parameter of considerable importance for distillate lubricating oils to be used in automotive engines and aircraft engines which are subject to wide variations in temperatures. This Index is a series of numbers ranging from 0 to 100 or more which indicate the rate of change of viscosity with temperature. A viscosity index of 100 indicates an oil that does not tend to become viscous at low temperature or become thin at high temperatures. Measurement of the kinematic viscosities of an oil at 40.degree. and 100.degree. C., and referral to established correlations, provides a measure of the V.I. of the oil. For purposes of the present invention, whenever V.I. is referred to it is meant the V.I. as determined by the ASTM Method D2270-77, published by ASTM, 1916 Race Street, Philadelphia 3, Pa., or equivalent, and accompanying tables, incorporated herein by reference.
To prepare high V.I. automotive and aircraft oils the refiner usually selects a crude oil relatively rich in paraffinic hydrocarbons, since experience has shown that crudes poor in paraffins, such as those commonly termed "naphthene-base" crudes yield little or no refined stock having a V.I. above about 40. (See Nelson, supra, pages 80-81 for classification of crude oils). Suitable stocks for high V.I. oils, however, also contain substantial quantities of waxes which result in solvent-refined lubricating oil stocks of high pour point, i.e. a pour point substantially greater than +25.degree. F. Thus, in general, the refining of crude oil to prepare acceptable high V.I. distillate stocks ordinarily includes dewaxing to reduce the pour point to not greater than +25.degree. F. The refiner, in this step, often produces saleable paraffin wax by-product, thus in part defraying the high cost of the dewaxing step.
Raw distillate lubricating oil stocks usually do not have a particularly high V.I. However, solvent-refining, as with furfural for example, in addition to removing unstable and sludge-forming components from the crude distillate, also removes components which adversely affect the V.I. Thus, a solvent refined stock prior to dewaxing usually has a V.I. well in excess of specifications. Dewaxing, on the other hand, removes paraffins which have a V.I. of about 200, and thus reduces the V.I. of the dewaxed stock. Minimal loss of V.I. on dewaxing is desirable.